1. Field of the Invention
The present invention relates generally to a buffer of a drive integrated circuit and, more particularly, to a buffer of a drive integrated circuit for driving a spatial light modulator that can meet a desired dynamic slew rate characteristic by controlling current that affects a slew rate.
2. Description of the Related Art
Generally, an optical signal processing technology has advantages in that a great amount of data is quickly processed in a parallel manner unlike a conventional digital information processing technology in which it is impossible to process a great amount of data in real time. Studies have been conducted on the design and production of a binary phase filter, an optical logic gate, a light amplifier, an image processing technique, an optical device, and a light modulator using a spatial light modulation theory.
The spatial light modulator is applied to optical memory, optical display device, printer, optical interconnection and hologram fields, and studies have been conducted to develop a display device employing it.
The spatial light modulator is embodied by a reflective deformable grating light modulator 10 as shown in FIG. 1. The modulator 10 is disclosed in U.S. Pat. No. 5,311,360 by Bloom et al. The modulator 10 includes a plurality of reflective deformable ribbons 18, which have reflective surface parts, are suspterminaled on an upper part of a silicon substrate 16, and are spaced apart from each other at regular intervals. An insulating layer 11 is deposited on the silicon substrate 16. Subsequently, a sacrificial silicon dioxide film 12 and a low-stress silicon nitride film 14 are deposited.
The nitride film 14 is patterned by the ribbons 18, and a portion of the silicon dioxide film 12 is etched, thereby maintaining the ribbons 18 on the oxide spacer layer 12 by a nitride frame 20.
In order to modulate light having a single wavelength of λ, the modulator is designed so that thicknesses of the ribbon 18 and oxide spacer 12 are each λ/4.
Limited by a vertical distance (d) between a reflective surface 22 of each ribbon 18 and a reflective surface of the substrate 16, a grating amplitude of the modulator 10 is controlled by applying voltage between the ribbon 18 (the reflective surface 22 of the ribbon 18 acting as a first electrode) and the substrate 16 (a conductive layer 24 formed on a lower side of the substrate 16 to act as a second electrode).
In an undeformed state of the light modulator with no voltage application, the grating amplitude is λ/2 while a total round-trip path difference between light beams reflected from the ribbon and substrate is λ. Thus, a phase of reflected light is reinforced.
Accordingly, in the undeformed state, the modulator 10 acts as a plane mirror when it reflects incident light. In FIG. 2, the reference numeral 20 denotes the incident light reflected by the modulator 10 in the undeformed state.
When proper voltage is applied between the ribbon 18 and substrate 16, the electrostatic force enables the ribbon 18 to move downward toward the surface of the substrate 16. At this time, the grating amplitude is changed to λ/4. The total round-trip path difference is a half of a wavelength, and light reflected from the deformed ribbon 18 and light reflected from the substrate 16 are subjected to destructive interference.
The modulator diffracts incident light 26 using the interference. In FIG. 3, reference numerals 28 and 30 denote light beams diffracted in +/−diffractive modes (D+1, D−1) in the deformed state, respectively.
However, the light modulator by Bloom adopts an electrostatic method to control the position of a micromirror, which is disadvantageous in that operation voltage is relatively high (usually 30 V or so) and the relationship between the applied voltage and displacement is not linear, thus resulting in poor reliability in the control of light.
In order to solve such problems, “thin-film piezoelectric light modulator and method of manufacturing the same” is disclosed in Korean Pat. No. P2003-077389.
FIG. 4 is a cut-away view showing a recess type thin-film piezoelectric light modulator according to a conventional technology.
Referring to the drawing, the recess type thin-film piezoelectric light modulator according to the conventional technology includes silicon substrates 401 and elements 410.
In this case, the elements 410 may have a predetermined width, and be regularly arranged to constitute the recess type thin-film piezoelectric light modulator. Alternatively, such elements 410 may have different widths, and be alternately arranged to constitute the recess type thin-film piezoelectric light modulator. The elements 410 may be positioned to be spaced apart from each other by a predetermined interval (almost the same as the width of an element 410). In this case, micromirror layers formed on the entire top surfaces of the silicon substrates 401 diffract incident light by reflecting the incident light.
Each of the silicon substrates 401 includes a recess to provide an air space to an element 410, an insulating layer 402 is deposited on the top surface of the silicon substrate 401, and both sides of the element 410 are attached to both sides of the silicon substrate 401 outside the recess.
The element 410 is formed in a bar shape, and both sides thereof are attached to both sides of the silicon substrate 401 outside the recess of the silicon substrate 401. The element 410 includes a lower support 411 the portion of which positioned above the recess of the silicon substrate 401 can move vertically.
The element 410 includes a lower electrode layer 412 formed on the left side of the lower support 411 and adapted to provide piezoelectric voltage, a piezoelectric material layer 413 formed on the lower electrode layer 412 and adapted to generate a vertical actuating force through shrinkage and expansion when voltage is applied to both sides thereof, and an upper electrode layer 414 formed on the piezoelectric material layer 413 and adapted to provide piezoelectric voltage to the piezoelectric material layer 413.
Furthermore, the element 410 includes a lower electrode layer 412′ formed on the right side of the lower support 411 and adapted to provide piezoelectric voltage, a piezoelectric material layer 413′ formed on the lower electrode layer 412′ and adapted to generate a vertical actuating force through shrinkage and expansion when voltage is applied to both sides thereof, and an upper electrode layer 414′ formed on the piezoelectric material layer 413′ and adapted to provide piezoelectric voltage to the piezoelectric material layer 413′.
A raised type light modulator different from the above-described recess type light modulator is described in detail in Korean Pat. No. P2003-077389.
Meanwhile, the drive Integrated Circuit (IC) of a spatial light modulator is used to drive the spatial light modulator. Generally, the slew rate of driving voltage is important to drive the spatial light modulator in light of the characteristics of the spatial light modulator. In this case, the slew rate refers to a maximum variation of output voltage per unit time. That is, when output voltage is plotted against time on a graph, the instantaneous slope (differentiated value with respect to time) of an output voltage curve is the slew rate.
Generally, the higher the slew rate, the better.
However, an excessively high slew rate in the drive IC for driving the spatial light modulator causes a large amount of residual vibration in the spatial light modulator, thus increasing fatigue in the spatial light modulator.
Furthermore, an excessively low slew rate in the drive IC for driving the spatial light modulator can solve the problem of fatigue caused by residual vibration, but is problematic in that it is difficult to obtain a desired number of diffracted beams having different intensities at desired time because the drive IC is driven so slowly. Generally, the spatial light modulator is desired to be driven fast. In the case where the slew rate of the drive IC is too low, it is problematic in that the drive IC cannot be driven at a desired speed.